In a previous issue of dvm360 ("Urine specific gravity measurement and interpretation in veterinary medicine," May 2013), we discussed the variables influencing normal specific gravity values. Here, we continue this review by discussing the pathophysiology of abnormal values.

Impaired urine concentration

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The interpretation of urine specific gravity values of randomly obtained samples depends on knowledge of the patient's hydration status and diet history, the plasma or serum concentration of urea nitrogen or creatinine, and knowledge of drugs or fluids that have been administered to the patient. Urine volume and water consumption may also be helpful. In some instances, interpretation may require serially performed evaluation of urine specific gravity on multiple samples. In others, evaluation of urine and plasma osmolality is needed.

Table 1: Differentiation of different forms of azotemia

If sufficient clinical evidence is present to warrant examination of the patient's renal function by determining the serum concentration of creatinine or blood urea nitrogen, the urine specific gravity (or osmolality) should be evaluated at the same time. Why? Because an adequately concentrated urine sample associated with an abnormal elevation in the serum creatinine or urea nitrogen concentration suggests the probability of prerenal azotemia, whereas intrarenal azotemia is probable in patients with elevated serum urea nitrogen and creatinine concentrations and less concentrated urine (Tables 1, 2 and 3).

Table 2: Urine specific gravity values

Varying degrees of impaired ability to concentrate or dilute glomerular filtrate are a consistent finding in all forms of primary renal failure but not all forms of renal disease. Because the kidneys have substantial functional reserve capacity, impairment of their ability to concentrate or dilute urine may not be detected until at least two-thirds (dogs) or more (cats) of the total population of nephrons has been damaged.

Complete inability of the nephrons to modify glomerular filtrate typically results in formation of urine with a specific gravity that is similar to that of glomerular filtrate (1.008 to 1.012). This phenomenon has commonly been called fixation of specific gravity. Once the ability to concentrate or dilute urine has been permanently destroyed, repeated evaluation of urine specific gravity will not help in the evaluation of progressive deterioration of renal function. Therefore, serial evaluation of urine specific gravity is of greatest aid in detecting functional changes earlier during the course of progressive primary renal failure or in monitoring functional recovery associated with reversible renal diseases.

Total loss of the ability to concentrate and dilute urine (specific gravity = 1.008 to 1.012) often does not occur as a sudden event but may develop gradually. For this reason, urine specific gravity values between about 1.007 to 1.029 in dogs and 1.007 to 1.039 in cats associated with azotemia are highly suggestive of primary renal failure, although, on occasion, hypoadrenocorticism may induce similar findings (Tables 1, 2 and 3). Likewise, urine specific gravity values between about 1.007 to 1.029 in dogs and 1.007 to 1.039 in cats that are clinically dehydrated but not azotemic are highly suggestive of primary renal failure or other disorders that impair urine concentrating capacity (Table 4).

If nonazotemic patients have impaired ability to concentrate urine, investigate causes of pathologic polyuria. If you determine the urine specific gravity or osmolality, it may allow you to determine whether a polyuric disorder characterized by water (1.001 ± 1,006) or solute (±1.008 or greater) diuresis is probable.